For assembly systems in the field of electronics manufacture, such as—for example—for automatic assembly systems for printed circuit boards, ever greater demands are being placed on the positional tolerances of the components due to the increasing miniaturization of components. The requirements in terms of assembly speed have likewise increased sharply in the last few years and will continue to increase still further in the future. These high expectations indicate that, in the future, assembly systems will require high-precision and rapid distance or height sensors as well as high-precision and rapid positional sensors for components. Such sensors will be used, for example, for checking the coplanarity of the terminal posts of the components. Distance or height sensors can also be used to control the distance between a component and the circuit substrate during the assembly process.
Until now, known devices for detecting height raster images including a multitude of three-dimensionally located points of object surfaces have essentially been based on the so-called triangulation process, in which a laser beam touches the surface of the object to be inspected. While the two planar location coordinates of a specific point on the surface area are known due to the current positioning of the laser beam, the height coordinate of the surface point currently to be measured is detected by at least one laterally positioned objective combined with a location-sensitive detector. Sequential illumination with the laser beam of the three-dimensional surface to be investigated thus enables the surface to be measured.
The resolution of optical distance sensors which use the triangulation procedure depends on the so-called triangulation angle. Since future miniaturization of components will also call for an improvement in the resolution of distance sensors, the triangulation angle in such optical distance sensors must be increased. However, this leads to a considerable increase in existing shadowing problems.
A further disadvantage of optical distance sensors based on the triangulation process is that the surfaces to be studied can have different optical diffusion factors. Precise distance measurements may only be possible for objects with surfaces that have an isotropic diffusion factor, i.e. a diffusion factor that is equally strong in all directions, for the laser beam as it falls on the surface. Such an isotropic diffusion factor is not usually guaranteed, particularly in the case of metallic reflective or even transparent surfaces.
A known method for the three-dimensional measurement of surface structures is based on the so-called confocal principle, in which a point-shaped light source, which is usually defined by an aperture plate, is imaged onto the surface of the object to be measured. The light backscattered by the surface is in turn imaged onto a virtually point-shaped detector, which is likewise usually defined by an aperture plate. The light falling on the detector is at maximum intensity when both the object level and the detector level are actually in the focal point of the respective lens. If the surface of the object to be investigated is outside the focal point, the measured beam is widened in front of the point-shaped detector and the measurable intensity decreases greatly.
A known sensor device for the optical measurement of distances according to the confocal principle is explained below with the help of FIG. 1. The sensor device 100 has a light source 101, a first aperture plate 102, a beam splitter 103, a second aperture plate 104, a light detector 105 and an objective 106. The first aperture plate 102 is arranged directly in front of the light source 101, so that the system comprising the light source 101 and the first aperture plate 102 functions as a point source of light, which has an effective light-emitting surface that corresponds to the cross-section of the opening in the first aperture plate 102. Correspondingly the second aperture plate 104 is arranged directly in front of the light detector 105. The light detector 105, the second aperture plate 104, the beam splitter 103, the objective 106 and the point that is currently being captured on the surface to be measured (not shown) lie on an axis which coincides with the optical axis of the objective 106.
The path of the beam in the sensor device 100 is explained below. The light emitted from the point source of light falls first on the beam splitter 103. This is positioned diagonally relative to the optical axis so that the light emitted from the point source, after being reflected off the beam splitter 103, is directed onto the objective 106, and is focused by this objective onto a focal point 107 which is located at least close to the surface to be measured. The light reflected back at least partially by the surface is in turn imaged onto the second aperture plate 104 by means of the objective 106. Only the light which is transmitted without further deflection by the beam splitter 103 is involved in this imaging process.
The two plates 102 and 104 are arranged confocally relative to the focal point of the objective 106, i.e. the distance between the second aperture plate 104 and the objective 106 is equal to the sum of the distances between the first aperture plate 102 and the beam splitter 103 and between the beam splitter 103 and the objective 106. For the optical imaging processes inside the sensor device 100, the aperture plate 102 is imaged onto the focal point 107 via reflection off the beam splitter 103 from the objective 106, and the focal point 107 is imaged onto the second aperture plate 104 by way of the objective 106.
The actual distance measurement takes place in that the entire sensor device 100 is displaced in direction z 108 relative to the surface to be measured (not shown). While the device is displaced, the light intensity measured by the light detector 105 is detected. The course 109 of the measured light intensity as a function of the distance between the sensor device 100 and the surface to be measured is drawn in the insertion 110.
A maximum level 111 appears precisely when the focal point 107 is lying directly on the surface to be measured. In other words, the maximum level 111 is achieved when the opening in the first aperture plate 102 is imaged onto the smallest possible area on the surface to be measured. In the confocal arrangement of the two aperture plates 102 and 104, the illuminated area on the surface to be measured (not shown) is imaged by way of the objective 106 onto the smallest possible area, which coincides with the opening in the second aperture plate 104. The distance from the corresponding point on the surface to be measured to the sensor device 100 can be determined from the course 109 of the light intensity, in particular from the precise position of the maximum level 111. An entire three-dimensional surface profile of an unknown structure is then determined by sequential distance measurements between the sensor device 100 and individual points on the surface to be measured.
According to the confocal principle the illumination and detection path are identical, i.e. the light falling on the surface to be measured and the light reflected by the surface to be measured run coaxially and shadowing phenomena can generally be disregarded.
Furthermore measurement of the distance by way of the sensor device 100 does not require the absolute value of the light intensity reflected back to be measured; the relative light intensity which is measured by the point-shaped light detector according to the displacement of the sensor device 100 in direction z 108 is sufficient. Thus any measurement of distance by the sensor device 100 is done almost without regard to the dispersal or reflection characteristics of the object surface to be measured. The use of a point-shaped light detector also has the effect that multiple reflections onto three-dimensional object surfaces do not lead to false measurements. A further important advantage of the confocal method is that it is highly accurate to sub-micrometers, which means that the accuracy requirements associated with the increasing miniaturization of components can easily be met.
One disadvantage of the sensor device 100 shown in FIG. 1 is that the entire sensor device 100 must be displaced relative to the surface to be measured in order to measure the distance. A further disadvantage is that several optical components must be used, and it has therefore not yet been possible, cost-effectively, to produce compact confocal sensors with small dimensions.
An optical distance sensor based on the confocal principle is known from WO 93/11403, and includes the following features: 1) the measuring beam is substantially larger than the lighting beam with regard to diameter, 2) the diameters of measuring beam and lighting beam are approximately equal at the point of measurement, whereby the lighting beam has a greater depth of focus than the measuring beam, and 3) a beam splitting unit with a number of beam splitters for dividing the measuring beam into a number of partial beams, with an approximately point-shaped photodetector arranged in each partial beam.
A further optical distance sensor based on the confocal principle is known from DE 19608468, with 1) a transmitting unit having a number of point-shaped light sources, which are imaged onto the surface of an object to be measured, 2) a receiving unit having a number of corresponding point-shaped light detectors in the same quantity, which are arranged confocally in the measurement region on the image side, whereby the point-shaped light sources and the corresponding point-shaped light detectors are arranged linearly in a plane which is orthogonal to the optical axis and generate a row of scanning points in a straight line on the surface of the object, 3) coaxial guidance of the illumination and measurement beams, and 4) a periodically variable optical path length between the receiving unit and the imaging optics.